FIG. 1 shows the waveforms of a motor driving current and a motor rotation detection signal in a prior art motor protection. In a conventional motor driving circuit, when the motor is restricted from rotating or becomes locked due to a foreign matter or a stuck rotor, the current supplied to the motor would continuously increase as a result of the locked motor. When the current is increased to a maximum limiting current value set by a current-limit circuit of the motor, the current is maintained at the maximum limiting current value. Then, when the motor is detected as failing to operate via a motor rotation detection signal, the power for driving the motor is cut off to prevent the windings inside the motor from being burned out and damaged due to overheat. And, after a period of time, the motor driving power is resumed and intermittently output, so as to test whether the motor has been released from the abnormal condition.
However, since the driving current limited by the maximum limiting current value set by the current-limit circuit is maintained at a high current the same as that for an initial starting of the motor, the resumed and intermittently output motor driving power would, due to the high current, cause excessive accumulation of heat in the windings inside the motor to result in constant and quick temperature raise in the windings and risks of a burn-out and damaged motor due to high temperature.
Moreover, whether the motor is in an abnormal condition is conventionally discriminated only by the motor rotation detection signal from a motor detection circuit. This type of motor protection does not necessarily meet the public demands for protecting a motor against abnormal condition.
Therefore, it is desirable to develop an improved motor protection device and method to overcome the shortcomings in the motor protection of prior art.